1. Field of the Invention
The present invention relates to an improved expanded foamed bead of a rubber-modified styrene polymer. More particularly, the present invention is concerned with an expanded foamed bead of a rubber-modified styrene polymer, comprising a plurality of closed cells defined by cell walls which constitute a matrix for the bead. Each of the cell walls comprises two surfaces separated by a distance equal to the thickness of the cell wall. The matrix comprises: (a) a continuous styrene polymer phase; and (b) a rubber phase dispersed in said continuous styrene polymer phase (a), wherein the rubber phase comprises a plurality of substantially flat, butadiene polymer rubber particles each having at least one styrene polymer particle occluded therein; wherein the flat rubber particles are arranged in a specific lamellar configuration along the thickness of the cell wall. The expanded foamed bead of a rubber-modified styrene polymer of the present invention is excellent in its ability to retain a foaming agent gas within the closed cells, and can advantageously be employed for producing, for example, packaging materials and cushioning materials or packings which are improved in resistance to breakage, such as cracking.
2. Discussion of Related Art
Expanded foamed beads of a styrene polymer are molded into various shaped products. During the molding, the expanded foamed beads are further foamed (or expanded) to give an ultimate foamed, molded product. The obtained foamed, molded products are widely employed as, for example, packaging materials and cushioning materials or packings. However, foamed, molded products produced from conventional expanded foamed beads of a styrene polymer are insufficient in resistance to breakage, such as cracking. Therefore, for example, when a foamed, molded product produced from conventional expanded foamed beads of a styrene polymer is used as a cushioning material in packaging an article having a relatively large weight and the packaged article is transported, there is a danger that when the packaged article sustains a jolt or impact during the transportation, the cushioning material suffers cracking. When the packaged article further repeatedly sustains an impact after the cracking of the cushioning material, it is likely that the article is damaged. The cracking of a cushioning material can be prevented by increasing the thickness of the cushioning material. However, when the thickness of a cushioning material to be used in packaging an article is increased, the volume of the resultant package becomes inevitably disadvantageously increased, so that the efficiency of the transportation is lowered.
In order to solve the above problems, it has been proposed to produce an expanded foamed bead from a styrene polymer having a butadiene polymer added thereto, i.e., a rubber-modified styrene polymer.
Unexamined Japanese Patent Application Laid-Open Specification No. 56-67344 discloses an expanded foamed bead of a styrene polymer having non-oriented rubber particles dispersed therein. In the above Japanese Patent document, a foamed, molded product produced from the expanded foamed beads of a rubber-modified styrene polymer disclosed therein is described to have an improved impact strength. However, since the rubber particles dispersed in the styrene polymer are of a non-oriented type, it is difficult for the rubber particles to change in conformity with the formation of cells during the expansion of the rubber-modified styrene polymer. Therefore, during the expansion of the rubber-modified styrene polymer, the rubber particles are likely to be disadvantageously exposed on the surfaces of cell walls or protrude into the cells, thereby adversely influencing the cells. Especially when a highly expanded foamed bead is produced, in which the thickness of cell walls is very small, the exposed or protruding rubber particles are likely to damage the cell walls, so that the retention of a foaming agent gas in the cells is unsatisfactory.
Unexamined Japanese Patent Application Laid-Open Specification No. 63-175043 discloses an expanded foamed bead which is uniform in cell size, and this foamed bead is made from a styrene polymer obtained by polymerizing a solution of a styrene-butadiene block copolymer in a styrene monomer. Unexamined Japanese Patent Application Laid-Open Specification No. 2-311542 discloses an expanded foamed bead improved in the strength of a molded product, in which the bead is made from a styrene polymer obtained by polymerizing a solution of a styrene-soluble rubber in styrene. However, foamed, molded products produced from the expanded foamed beads disclosed in these two Japanese Patent documents are unsatisfactory in resistance to cracking.
The cracking resistance of a foamed, molded product depends on various physical properties of the foamed, molded product, such as compressive strength, tensile strength and elongation, and the structure of cells in the foamed, molded product. These various physical properties of and the structure of cells in the foamed, molded product are influenced by the dispersion morphology of the rubber particles dispersed in the styrene polymer constituting the cell walls in expanded foamed beads used for producing the foamed, molded product.
Unexamined Japanese Patent Application Laid-Open Specification No. 3-182529 discloses an expanded foamed bead of a resin prepared by mechanically blending a high impact polystyrene and a hydrogenated styrenebutadiene block copolymer. However, when a rubber component is mechanically blended with a resin, the dispersion of the rubber component in the resin is likely to be uneven, so that the dispersion of the rubber component in the cell walls of an expanded foamed bead of the resin inevitably becomes uneven. The uneven dispersion of the rubber component in the cell walls is likely to cause breakage of the cell walls due to the presence of large aggregated rubber particles and form a large number of open cells, which allow a foaming agent gas to immediately escape therefrom. The breakage of the cell walls is especially prevalent in the case of a highly expanded foamed bead in which the thickness of the cell walls is small, so that the expanding capability of the expanded foamed bead is lowered. When an expanded foamed bead having a low expanding capability is subjected to molding under foaming, the resultant foamed, molded product inevitably has undesired voids between those beads, which are formed during the molding due to the unsatisfactory expansion of the expanded foamed beads. Such a molded product having voids has a poor appearance.